The present invention concerns apparatus for casting directionally solidified articles from a eutectic superalloy composition.
Superalloys are heat resistant materials having superior strength and oxidation resistance at high temperatures. Many of these alloys contain iron, nickel or cobalt alone or in combination as the principal element, together with chromium to impart surface stability and usually containing only one or more minor constituents such as molybdenum, tungsten, columbium, titanium and aluminum for the purpose of effecting strengthening. The physical properties of the superalloys make them particularly useful in the manufacture of gas turbine components.
The strength of superalloys is determined in part by their grain size. At low temperatures fine-grain equiaxed structures are preferred. At high temperatures large-grain size structures are usually found to be stronger than fine-grain size structures. It is believed that failure generally originates at grain boundaries oriented perpendicular to the direction of the induced stress.
An improved techniques for casting superalloys used in gas turbine engines is disclosed by F. L. Ver Snyder in U.S. Pat. No. 3,260,505, wherein a blade is formed having an elongated columnar structure with unidirectional crystals aligned substantially parallel to the long axis of the blade. This procedure involves directional solidification whereby almost a complete elimination of grain boundaries normal to the primary stress axis occurs. A further advance was made by B. J. Piearcy, as disclosed in U.S. Pat. No. 3,494,709 wherein grain boundaries in superalloys were eliminated by making single crystal castings.
Directional solidification to provide columnar casting and the apparatus used for this purpose are described on pages 479-508 in the text entitled The Superalloys, edited by C. T. Sims et al and published by John Wiley & Sons in 1972. Columnar grains are formed when the melt temperature is greater than the freezing temperature and when the flow of heat is unidirectional from the liquid through the solid. Typically, a ceramic investment casting mold is attached to a water-cooled copper chill plate and placed in an induction-heated graphite susceptor. The mold is heated above the melting point of the alloy being cast and the superheated melt is poured into the mold. Heat enters the upper portion of the mold by radiation from the susceptor and is removed through the solidified metal by the chill at the bottom. Thus, solidification occurs in an upward direction through the casting, and the rate of solidification is a function of the amount of heat entering at the top of the casting and the amount of heat extracted from the casting through the solid.
In the Stockbarger method described in the aforementioned text, the furnace heat-flow configuration requires a sharp temperature difference between the lower and upper furnace portions which is provided by a baffle at the bottom of the furnace. The mold is gradually withdrawn through the baffle so that the solid-liquid interface remains essentially parallel with the plane of the baffle.
The Bridgman-type apparatus has been used to produce acceptable elongated grain structures of numerous superalloys. Here the susceptor is heated inductively, which melts the charge in the crucible. After equilibrium is established, the mold assembly is lowered out of the heat zone and nucleation of solid occurs in the bottom of the crucible. Directional freezing continues upward as the mold unit is lowered.
Walter et al, in U.S. Pat. No. 3,793,012, discloses the preparation of unidirectionally solidified nickel-base carbide reinforced cast superalloy bodies having high strength and high stress-rupture properties, particularly at elevated temperatures. The reinforced fibers present in the matrix were aligned single crystal fibers of metal monocarbides. When such castings are made in shell molds, as in the manufacture of turbine blades, the outer configuration of the shell is not symmetrical due to projections of the platform and dovetail sections, as well as the thinning and twisting of the shell to conform to the pattern of the airfoil section of the blade. Attempts to directionally solidify a blade using such an irregular shaped shell have proven very difficult due to the gap between the radiation baffle and the shell to allow clearance of the shell as it is being withdrawn from the heat zone.
Turbine blades as they are presently processed, possess acceptable fiber alignment throughout the length of the airfoil. However, as the solid-liquid interface approaches the vicinity of the blade root in the solidification process, a breakdown to cellular morphology occurs. One of the reasons for the breakdown is reported to be due to an uncontrolled varying solidification rate, even though the casting is lowered out of a hot furnace at a constant rate. This non-uniform growth rate is believed to be the result of a varying heat input and heat output of the casting since, as mentioned earlier, the mold varies in cross section and must pass through a constant cross section baffle. When the platform of a turbine blade casting enters the baffle, more heat is applied to the casting since the gap between the mold and baffle decreases, resulting in a slowdown of the advancing solid-liquid interface.
In U.S. Pat. No. 3,942,581 issued to T. F. Sawyer on Mar. 9, 1976 there is disclosed an improved apparatus for producing a directionally solidified article from a eutectic superalloy composition. The apparatus comprises a mold within a heating zone in which the bottom of the mold is attached to a chill plate. A retaining means for a ceramic insulation surrounds the heating zone and the lower portion of the mold, the retaining means being attached at its bottom to a chill plate; A hollow bubble type ceramic insulation is disposed in the retaining means and is in contact with the lower portion of the mold. Also disclosed is a means for lowering the mold out of the heating zone and to permit the insulation to form a continuous heat barrier around the portion of the mold descending out of the heating zone.
The Sawyer patent, supra, further discloses a method of using the apparatus to directionally solidify nickel-or-cobalt base superalloys by disposing a plurality of hollow ceramic bodies around the mold such that as the mold is lowered, the bodies conform to and form a contacting layer around the outside surface of the mold as it is exposed below the heating zone, whereby lateral heat transfer is prevented.
An undesireable feature of the device disclosed and claimed by the Sawyer patent supra, is that the hollow spherical ceramic insulation approaches the mold from a jacket surrounding the susceptor and therefore, the insulation takes a path adjacent the hot parts of the furnace. Sintering of the insulation material may occur at high temperatures if it is allowed to come into contact with the hot sections of the furnace. As a result, the insulation, may become a coherent mass instead of individual bubbles, be unable to flow and thereby fail to provide a radiation barrier about the mold as it is lowered out of the furnace. In addition, if the article to be cast has a high ridge, the individual insulative spheres may not abut the areas of the mold near the ridge.